Reduce spam user sign-ups

Why are we doing this?

Prior work: https://github.com/opensafely-core/opencodelists/issues/2113

Spam users are signing up for OpenCodelists again - causing issues when selecting a reviewer, and adding unnecessary accounts to the site.

How will we know when it's done?

We reduce the amount of spam users who can sign-up

What are we doing?

A few options:

1. Add email validation by making a user confirm their account with a link
2. Add Cloudflare protection to the sign-up page (click to view page)
3. Add Cloudflare Turnstile as a captcha to the sign-up form
4. Instead of reducing spam users, change the selecting a reviewer workflow to make it easier to find the user that you want. Maybe we can reduce the list to members of an organisation, though that would need thought. Or maybe we can make it easier/more obvious how to use the drop-down effectively.

Defining delivery tasks guidance

Start with prior issue #2113 and the sign-up page to understand the spam problem and compare it with the reviewer-selection workflow. Choose and scope one of the proposed approaches before implementation; done means spam sign-ups are reduced without leaving the reviewer workflow unresolved.